Interpretive Summary: As a signatory of the United Nations Framework Convention on Climate Change, the US is required to report national greenhouse gas (GHG) emissions for different sectors of the economy. Nitrous oxide (N2O) is an important GHG and agriculture is its primary anthropogenic source. Until recently, emission factor methodology been used to estimate N2O emissions from the agricultural sector. Emission factor methodology is based solely on nitrogen (N) inputs (e.g., fertilizer, manure) and does not account for other factors, such as climate and soil type, that also influence N2O fluxes. To account for some of these other factors, the 2005 EPA GHG inventory includes estimates of N2O emissions from cropped soils using DAYCENT, a process based computer model. DAYCENT simulates plant growth and the microbial processes in soil that result in N2O emissions. DAYCENT simulated emissions from major crops in the US (corn, soy, wheat, hay, cotton, sorghum) and simpler emission factor methodology was used to estimate emissions from remaining cropped land. N2O emissions from simulations of presettlement native vegetation were subtracted from cropped soil N2O to isolate anthropogenic emissions. Weather, soils, and land management information are required to run DAYCENT. Weather data and dominant soil texture class that lie closest to the geographical center of the largest cluster of cropped land in each county were selected to drive DAYCENT. Land management information was implemented at roughly the state level. Maps of model simulated county level crop yields were compared with yields estimated by USDA for quality control. Combining results from DAYCENT simulations of major crops and emission factor methodology for remaining cropland yielded an estimate of ~179 Tg CO2 equivalents for mean annual anthropogenic N2O emissions for 1990-2003.

Technical Abstract:
Until recently, Intergovernmental Panel on Climate Change (IPCC) emission factor methodology, based on simple empirical relationships, has been used to estimate carbon (C) and nitrogen (N) fluxes for regional and national inventories. However, the 2005 US Environmental Protection Agency (EPA) greenhouse gas inventory includes estimates of N2O emissions from cultivated soils derived from simulations using DAYCENT, a process-based biogeochemical model. DAYCENT simulated major US crops at county-level resolution and IPCC emission factor methodology was used to estimate emissions for the approximately 14% of cropped land not simulated by DAYCENT. The methodology used to combine DAYCENT simulations and IPCC methodology to estimate direct and indirect N2O emissions is described in detail. N2O emissions from simulations of presettlement native vegetation were subtracted from cropped soil N2O to isolate anthropogenic emissions. Meteorological data required to drive DAYCENT were acquired from DAYMET, an algorithm that uses weather station data and accounts for topography to predict daily temperature and precipitation at 1 km2 resolution. Soils data were acquired from State Soil Geographic Database (STATSGO). Weather data and dominant soil texture class that lie closest to the geographical center of the largest cluster of cropped land in each county were used to drive DAYCENT. Land management information was implemented at the agricultural/economic region level, as defined by the Agricultural Sector Model. Maps of model-simulated county-level crop yields were compared with yields estimated by the US Department of Agriculture (USDA) for quality control. Combining results from DAYCENT simulations of major crops and IPCC methodology for remaining cropland yielded estimates of approximately 109 and approximately 70 Tg CO2 equivalents for direct and indirect, respectively, mean annual anthropogenic N2O emissions for 1990-2003.